The saline lake contains many mineral salts and is an important source of mineral salts. However, large-scale exploitation of saline lake is difficult because the content of mineral salts in the saline lake is generally low. The main mining technologies of recent years are evaporation of saline lake water in the sun to obtain salt, deposit mining and the like, in which evaporation of saline lake water in the sun is the most environment-friendly exploitation method. Further development is seriously restricted because of its low efficiency. Therefore, a method which can improve the efficiency of the evaporation crystallization will have important strategic value.
Lithium is a metallic element having the lowest atomic weight. It possesses special properties such as the most negative potential, the greatest electrochemical equivalent, high specific heat, high conductivity, and strong chemical reactivity etc. It is an ideal metallic material for manufacturing disposable batteries, rechargeable power batteries, and structural material for aerospace etc. Thus, it is honored as the energy metal in the 21st century.
In nature, lithium exists mainly in the form of pegmatite such as spodumene, lepidolite etc., and in the form of lithium ions in the saline lake brine, underground brine and seawater. The saline lake lithium resource reserves account for more than 69% of the world industrial lithium reserves, while the saline lake lithium resource reserves of China account for 85% of the industrial lithium reserves. Extracting lithium from saline lake brine possesses features such as simple technology, low energy consumption, and low cost. It has gradually substituted the production of lithium from lithium ore. Presently, there are many ways to extract lithium from saline lake, for example, ion exchange, adsorption, carbonization, precipitation, and extraction. However, these methods are immature, costly, or cannot meet the environmental requirements, thus, making it difficult to realize industrial production.
The current industrial production of lithium salt from saline lake in the country mostly employs salt-pan process, including enriching the lithium by brine evaporation in the drying bed in the winter, then pouring the lithium-rich brine into a solar pond for heat accumulation and retention until the temperature reaches between 30° C. and 50° C. More lithium carbonates reach oversaturation and precipitate as the solubility of lithium carbonate in brine is decreased with increasing temperature. Then, upon further chemical processing, the industrial grade of lithium carbonate is obtained. Although this process takes advantage of the superiority of plateau solar energy and cold energy, its production cycle is too long (the brine evaporation stage takes 4 to 6 months, the crystallization stage takes 2 to 4 months), leading to only one harvest a year, and the production efficiency is very low, leading to little benefit. Moreover, the construction of the solar pond requires lots of costs and materials, the geomembrane for heat preservation laying at the bottom of the pond is costly despite having poor leakage resistance. Therefore, a significant loss of brine results from the leakage problem of the geomembrane and it adversely affects the productivity and economic benefits.
The constituent of brine is very complex, having low content of lithium as compared to that of sodium and potassium. Therefore, it is necessary to concentrate the lithium before extraction so as to improve the grade of the lithium salt. The solubility of lithium carbonate in water decreases with temperature increase. This is referred to as an inverse solubility property. Experiment indicates that the solubility of lithium carbonate in brine has a similar property, while sodium salt and potassium salt have a positive solubility property. Therefore, the freezing method can be used to precipitate a large amount of sodium salt and potassium salt at low temperature, causing the preliminary enrichment of lithium in brine, then directly heat the lithium-rich brine to achieve the purpose of extracting high-grade lithium carbonate. The relational experiments show that sodium salt and potassium salt may precipitate out more quickly by quick-freezing than by general-freezing. Therefore, a higher enrichment efficiency of lithium can be achieved by using quick-freezing. If the concentration of lithium carbonate in brine has not reached saturation concentration at a certain temperature, then even if the temperature of the brine is higher, the lithium carbonate will not crystallize. As a result, it will have to be kept warm longer to allow the lithium salt reach saturation concentration and precipitate out. In order to shorten the holding time and accelerate the enrichment of lithium in brine, a reduced-pressure evaporation can be used.
Reduced-pressure evaporation is an efficient way for increasing evaporation rate. With regard to a water-containing solution or a volatile solvent-containing solution in a sealed container, the equilibrium between the liquid phase and the gas phase is balanced. When the system pressure decreases, i.e., after the gas phase on the surface of the liquid is removed, the equilibrium between the gas phase and the liquid phase is broken. Then, the water or volatile solute in the liquid phase tends to evaporate to supplement the gas phase so as to maintain the gas-liquid equilibrium. Thereby the boiling point of the solution decreases. The liquid phase will keep boiling and evaporating if the evaporated gas is continuously removed, and the solute will get continuously concentrated. When the solute concentration reaches the saturation concentration of this temperature, the solute will crystallize.
Many devices and methods have been reported for extracting Li2CO3 from saline lake brine both at home and abroad, for example, “Adsorption Techniques and Progress on the Extraction of Lithium from Salt Lake Brines” (Journal of Salt and Chemical Industry, vol. 36(3), 2007), “Preparation of Lithium Carbonate from Brines by Solvent Extraction” (Journal of Salt lake Research, vol. 14(2), 2006), “Progresses on the Process and Technique of Lithium Recovery from Salt Lake Brines Around the World” (World Sci-Tech R & D, vol. 28(5), 2006), “The Investigation on Lithium in the Bitterns of Enrichment by Solvent Flotation” (Journal of Salt and Chemical Industry, vol. 40(1), 2011) and the like. These devices and methods have the advantages that the adsorption method takes advantage of an ionic sieve type oxide by using its good selective adsorption property towards lithium characterized by a selective coefficient that can reach up to 104 to 105. Thus, it can be effectively used to extract lithium from saline lake brine, and its mechanical strength and chemical stability are excellent. Solvent extraction is suitable for saline lake brine having higher content of magnesium chloride, characterized by its simple technology, high product purity, and lower energy consumption. The carbonate precipitation method is a simple process, with high reliability. It is suitable for saline lake brine having a low ratio of magnesium to lithium. The disadvantages of the methods mentioned above are that: there are great differences between the practical and the theoretical adsorptive capacity of the ionic sieve type adsorbent; the poor permeability and difficulty to granulate restricts the industrial application of this method; the device is susceptible to corrosion; the loss of extracting agent is great in solvent extraction method; and low extraction efficiency in the general precipitation method. None of the methods mentioned above can achieve higher productivity while protecting the environment.
There still exists a need in the art to get a crystallization method and a device for effectively extracting lithium salt from saline lake brine which are also environment-friendly.